This invention relates to motion and positional control of magnetic tape in a reel-to-reel tape drive in which the magnetic tape can be transported hi-directionally for recording and playback of information in either tape direction.
The control of magnetic tape motion and position in reel-to-reel tape drives is described in detail in U.S. Pat. Nos. 4,015,799, and 4,125,881, both assigned to the assignee of this application and incorporated herein by reference in their entireties.
U. S. Pat. No. 4,015,799 relates to the use of a finely graduated ("fine line") tachometer on an idler roller engaging a magnetic tape to measure the amount of tape being advanced during a complete revolution of each tape reel shaft in a reel-to-reel tape drive system. The amount of tape advanced is converted to the radius of each reel once each revolution of each reel. Reel radius is then used to determine drive currents for each reel motor so as to provide a precise control of tape position and motion.
U.S. Pat. No. 4,125,881 describes a reel-to-reel tape drive in which magnetic tape is moved from one reel to another, passing a read/write head. A fine-line tachometer is mounted on one reel shaft to provide a fine-line tachometer reading in the form of a number of pulses per revolution. A second tachometer on the second reel shaft provides a single pulse per revolution of the second reel. The single pulse is used to gate the counting of fine-line tachometer pulses for each revolution of the second reel. A servo algorithm uses the gated per-revolution fine-line tachometer count to determine the reel radii based upon the actual length and thickness of the magnetic tape whose position and motion the servo system controls. Motor acceleration currents of a magnitude corresponding to the reel radii are generated to drive the reel motors.
Both of these incorporated patents are concerned with uni-directional tape drives in which magnetic tape is written and read in one direction. No recording occurs during movement of the tape in the opposite direction, which is used only for rewinding and repositioning the tape. However, in a bi-directional tape drive in which the magnetic tape can be recorded in either direction, the tape servo algorithm of the '881 patent cannot accurately determine the radius of the tape reel and position of data on the tape when the direction of tape writing is reversed.
The limitation in the servo algorithm of the '881 patent stems from air entrainment between the outermost layers of tape on the take-up reel. It has been determined that movement of the tape during writing creates a relatively thin air bearing between the undersurface of the portion of the tape traveling between the write/read location and the take-up reel. The air bearing is entrained in the outermost layers of tape on the take-up reel, being dissipated thereafter when the air trapped between layers of tape on the take-up reel escapes. This problem went unnoticed in the tape drive described in the '881 patent because the take-up reel provided only the index pulse during tape writing, with the fine-line tachometer pulses being provided from the supply reel. Since air is not entrained on the supply reel, fine-line pulses correlate very accurately with the radius of the tape remaining on the supply reel. However, if the write direction were reversed in the reel-to-reel drive of the '881 patent (assuming the drive to be reversible), the fine-line tachometer pulses now generated from the take-up reel would not correlate as accurately with the tape radius and tape position on the take-up reel. The loss in accuracy would significantly degrade tape drive operation when writing multiple data records separated by inter-block gaps.
While writing multiple data records at constant tape velocity, inter-block gaps (IBGs) between records are generated by timing the interval traveled between the records. This produces a well-controlled IBG whose size is determined by the tape speed and the time interval period. In order to maximize tape cartridge capacity, IBG size is minimized.
When the writing process stops due to an interruption of data available from a host system or a write data buffer, the tape drive must stop the tape and await the next write operation. Because of the very short length of the IBG and the relatively long stop and start distance required for the tape drive to accelerate, the tape drive motion servo system executes a "back hitch" in which tape motion is slowed following writing of the IBG, stopped, and then reversed back to a point where the write/read head precedes the location of the last-written data. When the writing process begins again, the tape is accelerated from its stop position up to the constant write velocity at which time the last data record and the IBG immediately following it have passed the write/read head and the next record is written.
In executing the back hitch operation, the position of the last-written data record on the tape relative to the write/read head is controlled by the tape motion servo system by using the output of a fine-line tachometer and by measuring timing between the end of the last-written data and a particular fine-line tachometer pulse. To start the back hitch, the data channel issues a synchronizing signal to the tape motion servo system indicating the end of the last data record. The tape motion servo system measures and stores the time lapse between this synchronizing signal and the next fine-line tachometer pulse which occurs (which becomes a position reference pulse). This time is subtracted from the desired IBG transit time to produce a time reference or partial IBG time for use in resynchronizing the recording channel circuits to the last data record on the tape. The fine-line tachometer pulses are counted for the purpose of locating the position reference pulse after the back hitch motion has been executed. When the position reference pulse is located, a write start point is achieved, and the tape motion servo system times out the remaining partial IBG time, issuing a resynchronizing signal to the data channel when the timeout completes. The resynchronizing signal thus occurs at the end of a nominal IBG distance from the previously-written data record, and a new data record is appended.
The accuracy of the process of resynchronization during the back hitch operation is limited by the integrity of the fine-line tachometer pulses. In particular, the correspondence between the fine-line tachometer pulses and the position of the data on the tape relative to the write/read head is dependent on the radius of the tape stack of the reel on which the fine-line tachometer is mounted. The tachometer pulses provide a measurement of the angular position of the reel which corresponds by radius to linear position of the tape. On the take-up reel, air entrainment increases the apparent radius of the tape stack, thereby compromising the integrity of the correspondence between the stack of tape on the reel and the reel hub. Therefore, the integrity of the correspondence between the fine-line tachometer pulse count and the radius of the tape stack is degraded if the fine-line tachometer output is obtained from the take-up reel. For very short IBGs, which are required to maximize data capacity, controlled IBG positioning must be accomplished with the supply reel. Thus, for a tape drive which writes in both directions, limiting the fine-line tachometer to only one reel, as taught in the '881 patent, introduces the potential for loss of data if the disparity between the actual and apparent tape stack radius is large enough.
The IBM 3480 tape drive product, which embodies the invention described and claimed in the '881 patent, is a reel-to-reel unit which utilizes a single-reel tape cartridge of a type described, for example, in co-pending U.S. patent application Ser. No. filed on Jun. 14, 1993 for "Magnetic Tape Cartridge with Second Generation Leader Block and Leader Block Pin" by G. S. M. Robles et al. which is incorporated herein by reference in its entirety. When the cartridge is initially loaded into the tape drive, it is placed in engagement with a tape drive reel. When the cartridge is loaded, a leader block mounted to, the tape's leading end is engaged by a threading mechanism which pulls the tape by the leader block around a threading path to a take-up reel which has a notch for receiving the leader block. During the threading process, the notch on the take-up reel must be precisely positioned at a point where the threading mechanism places the leader block into the notch. In the prior art IBM 3480 tape drive, two position sensors located on the take-up reel assembly detect and provide indication of the take-up reel position during threading. These two sensors are separate, non-integrated units which increase the expense and complexity of the reel-to-reel tape drive.
Manifestly, there is a need in a reversible reel-to-reel tape drive for solutions to the air entrainment problem and to the problem of precisely positioning the take-up reel during threading.